1. Field of the Invention
The present invention relates to a method of controlling a liquid crystal display panel (LCD panel), and more particularly to a driving method and a circuit for recycling charges on display electrodes using adjacent scan line control signals, thereby reducing the power consumption and lowering the time required for charging and discharging of the display electrode.
2. Description of the Related Art
FIG. 1 (PRIOR ART) schematically shows an equivalent circuit of the conventional thin film transistor liquid display panel (hereafter referred to as TFT-LCD). As shown in the figure, the liquid crystal display panel is composed of the cross-connected data electrodes (D1, D2, D3, . . . and Dy) and the scan electrodes (G1, G2, . . . and Gx), each pair of the data electrodes and the scan electrodes can be used to control a display cell. For example, data electrode D1 and scan electrode G1 can be used to control the display cell 100. As shown in the figure, the equivalent circuit of display cell 100 (the same for other display cells) include a thin film transistor 10 for controlling, a storage capacitor Cs and a liquid crystal capacitor Clc constructed by the display electrode and the common electrode. The gate and the drain of the thin film transistor 10 are connected to scan electrode G1 and data electrode D1 respectively. The video signal carried by the data electrode D1 can be written to the display cell 100 by controlling the conducting state of the thin film transistor 10 using the scan signal carried by the scan electrode G1.
Scan driver 3 sends out the scan signal on the scan electrode G1, G2, . . . Gx sequentially, according to scan control signals. When one of the scan electrode is scanned, the thin film transistors corresponding to this scanned scan electrode are turned on and the thin film transistors corresponding to other scan electrodes are turned off. When the thin film transistors of the display cells on a row are turned on, data driver 2 sends corresponding video signal (gray level) to y display cells on the row through data electrode D1, D2, and Dy according to the image data to be displayed. When scan driver 3 finishes the scanning of the x scan lines, the display of a single video frame is done. The scanning of the scan lines described above is performed repeatedly, thereby displaying subsequent video frames.
According to the relationship between the common electrode voltage VCOM and the sent video signals on the data electrode, the polarities of the sent video signals on the data electrodes can be positive or negative relative to the common electrode voltage VCOM. FIG. 2 (PRIOR ART) schematically shows the relationship between the common electrode voltage VCOM and the video signals of different polarities. As shown in FIG. 2, the positive video signals are positioned between common electrode voltage VCOM and the system high voltage VDD. According to the gray level represented by the positive video signal, the actual voltage is positioned between voltages Vp1 and Vp2 (in general, the closer the positive video signal to the common electrode voltage, the lower its gray level). In contrast, the negative video signals are positioned between common electrode voltage VCOM and the system low voltage VSS. According to the gray level represented by the negative video signal, the actual voltage is positioned between voltage Vn1 and Vn2 (Similarly, the closer the negative video signal to the common electrode voltage, the lower the gray level it corresponds to). When the positive video signal and the negative video signal corresponding to the same gray level have the same visual effect theoretically.
To prevent the liquid crystal molecules being subjected to a voltage bias of single polarity and therefore shortening the life of the liquid crystal molecules, a single display cell in the general TFT-LCD is driven by video signals of opposite polarities in the odd-numbered video frames and even-numbered video frames.
There are four driving schemes to achieve the above-described requirement, including frame-inversion, line-inversion, column-inversion and dot-inversion.
FIG. 3A (PRIOR ART) shows the pattern of the polarities of the video signals received by the display cells in the frame-inversion driving scheme. As shown in FIG. 3A, two diagrams show the patterns of the polarities of the video signals received by each display cell in the area defined by data electrodes Dnxe2x88x921, Dn, Dn+1 and scan electrodes Gmxe2x88x921, Gm, Gm+1 in an odd-numbered video frame and an even-numbered video frame, respectively. In the left diagram, which-is corresponding to the odd-number video frame, all video signals are positive (denoted by xe2x80x9c+xe2x80x9d). On the other hand, in the right diagram corresponding to the even-numbered video frame, all video signals are negative (denoted by xe2x80x9cxe2x88x92xe2x80x9d). The frame-inversion driving scheme uses video signals of different polarities in adjacent video frames for all display cells.
FIG. 3B (PRIOR ART) shows the pattern of the polarities of the video signals in the line-inversion driving scheme. The difference between FIG. 3A and FIG. 3B lies in that the display cells of the same row (the same data line) in the same video frame receive video signals of the same polarity, however the display cells of the adjacent rows receive video signals of the opposite polarity.
FIG. 3C (PRIOR ART) shows the pattern of the polarities of the video signals in the column-inversion driving scheme. The arrangement of the video signal polarities in FIG. 3C is similar to that in FIG. 3B. The display cells of the same columns (the same data lines) in the same video frame receive the video signals of the same polarity, and the display cells of the adjacent columns receive video signals of the opposite polarity.
FIG. 3D (PRIOR ART) shows the pattern of the polarities of the video signals in the dot-inversion driving scheme. In the dot-inversion driving scheme, if one display cell is driven by the positive video signal, the four display cells adjacent to this display cell are driven by the negative video signals.
FIG. 4 (PRIOR ART) shows a circuit diagram of a portion of the conventional liquid crystal display panel, including data electrodes (Dnxe2x88x921, Dn and Dn+1), scan electrodes (Gmxe2x88x921, Gm) and the corresponding display cells. When the scan signal appears on the scan electrode Gmxe2x88x921, the thin film transistors connected to scan electrode Gmxe2x88x921 are turned on and the video signals on data electrodes Dn and Dn+1 can be coupled to the display electrodes of the corresponding display cells. When the scan signal appears on scan electrode Gm, thin film transistors TFT1 and TFT2 connected to scan electrode Gm are turned on and the video signals on data electrodes Dn and Dn+1 can be coupled to the display electrodes P1 and P2 of the corresponding display cells.
Assume that the circuit shown in FIG. 4 adopts the dot-inversion or column-inversion driving scheme for determining the polarities of various video signals. FIG. 5 (PRIOR ART) shows a timing diagram of the signals in display electrodes P1 and P2 and scan electrodes Gmxe2x88x921 and Gm. Pulses 20 and 21 in the scan electrode Gmxe2x88x921 indicate the scanning of the scanning line corresponding to scan electrode Gmxe2x88x921 in two adjacent video frames, respectively. Pulses 30 and 31 in the scan electrode Gm indicate the scanning of the scanning line corresponding to scan electrode Gm in the corresponding video frames, respectively. Each scan signals 20, 21, 30 and 31 can turn on the connected thin film transistors, thereby coupling the video signals on the data electrodes to the corresponding display electrodes.
The operation is described as follows by using scan electrode Gm as an example. Before the scan signal 30 is sent (before time t1), the video signal coupled to the display electrode P1 is positive (between voltages Vp1 and Vp2), and the video signal coupled to the display electrode P2 is negative (between voltages Vn1 and Vn2).
During the period of the scan signal 30 (t1xcx9ct2), the scan signal 30 turns on thin film transistors TFT1 and TFT2, and video signals couple to display electrode P1 and P2 through data electrodes Dn and Dn+1. As mentioned above, the video signal coupled to one display cell in one video frame has a polarity opposite to that of the video signal coupled to the same display cell in the previous video frame.
Therefore, during the period t2xcx9ct3, the thin film transistors TFT1 and TFT2 are turned off. The video signal coupled to the display electrode P1 is negative in polarity (between voltages Vn1 and Vn2), and the video signal coupled to the display electrode P2 is positive (between voltages Vp1 and Vp2).
After completing the scanning of other scan lines (Gm+1xcx9cGx, G1xcx9cGmxe2x88x921), the scan signal 31 corresponding to next video frame is sent to the scan electrode Gm (time t3-t4). At this time, the polarity of the video signal is opposite to that in the previous video frame. In other words, the video signal coupled to display electrode P1 is positive (between voltages Vp1 and Vp2), and the video signal coupled to display electrode P2 is negative (between voltages Vn1 and Vn2). Therefore, the video signals of opposite polarities are sequentially sent to the corresponding display cells in adjacent odd-numbered video frames and even-numbered video frames.
However, there is a drawback in the conventional driving schemes. More specifically, since the video signals sent out by data electrodes change either from the positive polarity to the negative polarity or from the negative polarity to the positive polarity, the driving schemes require a lot of power and too much heat is generated.
U.S. Pat. No. 6,064,363 disclosed a technique of recycling charges to reduce the violent variation of voltage levels in these video signals. A recycling cell is designed for adjacent two data electrodes. The recycling cell is controlled by control signals generated by additionally control circuitry. It can recycle the charges on the adjacent data electrodes before one of the scan lines is scanned (i.e. for all of the display cells in the same scan line), and evenly distributes them. Since the adjacent display cells receive video signals of different polarities in the dot-inversion driving scheme, the voltage on the data electrodes can approach to the common electrode voltage VCOM after the charges are recycling and evenly distributed. Driving these data electrodes only requires driving to a level corresponding to the positive or negative polarity from the common electrode voltage VCOM, thereby reducing power consumption.
However, the ""363 patent also has its disadvantages. First, the ""363 patent employs the independent control signal to control the above-mentioned charge-recycling mechanism, and, therefore, an extra control circuit is required to generate the corresponding control signal. Furthermore, in the ""363 patent, the charges on the adjacent data electrodes are recycled and re-distributed for every scanning line. In addition, since the thin film transistors of the display cells are turned off during the charge-recycling stage, the charges on the display electrodes are not recycled and the charges induced in the last video frame still remain in the display electrodes. In fact, when the data driver couples the video signals to the corresponding display cells, it should drive data electrodes as well as the display electrodes inside the display cells. Hence, the data driver still drives the display electrodes between positive video signal levels and negative video signal in a very short period. Obviously, the power-consumption problem cannot be completely solved by the mechanism proposed by the ""363 patent.
An object of the present invention is to provide a novel liquid crystal display panel and the control method thereof, which do not need extra control circuitry to perform the charge recycling and redistribution process for charges resided in the data electrodes and coupled within the display electrodes. Therefore, the power consumption is reduced.
The liquid crystal display panel of the present invention is controlled and driven by data drivers and scanning drivers. The video signals from the data drivers are in the dot-inversion or column-inversion driving scheme. In other words, the video signals from two adjacent data lines are of opposite polarities. The liquid crystal display panel includes a first data electrode and a second data electrode to receive video signals of different polarities. The first data electrode and the second data electrode, together with a scan electrode crossing them, correspond to a first display cell and a second display cell, respectively. Every display cell has its own display electrode and the control transistor. In addition, the gate of the control transistor is coupled to the scan electrode, and the source and the drain of the control transistor are coupled between the display electrode and the corresponding data electrode. The video signal on the data electrode is coupled to the internal display electrode under the control of the scanning signal on the scan electrode. There is a recycling transistor located between the first display cell and the second display cell. The gate of the recycling transistor is coupled to another scanning electrode which is preferably adjacent to the present scan electrode and scanned earlier than the present scan electrode. The source and the drain of this recycling transistor are coupled between the display electrodes of the first display cell and the second display cell. In other words, when the adjacent scan electrode is scanned previously, the recycling transistor can be turned on to redistribute the charges in the display electrodes of the first display cell and the second display cell.
Since the polarities of the original video signals are opposite, the resulted voltage on the display electrodes of the first display cell and the second display cell is very close to the common electrode voltage (VCOM) after recycling and redistributing. Therefore, the power consumption is reduced. In fact, the display electrode of each display cell can be coupled to the display electrodes of adjacent display cells on the same scan line by means of two recycling transistors.
In addition, the present invention also provides a method of controlling a liquid crystal display panel. The scanning driver sends scanning signals to the individual scanning electrodes sequentially to turn on the control transistors of all the display cells on the corresponding scanning electrodes. Meanwhile, the data driver sends the corresponding video signals to the data electrodes. These video signals are coupled to the display electrodes of the display cells through the turned-on control transistors. In addition, these scanning signals can be used to control an device, which can be used to conduct display electrodes of adjacent display cells on a scanning electrode different from the present scanning electrode. Therefore, charges on the display electrodes and data electrodes can be evenly redistributed and the power for driving the video signals to the display electrodes can be reduced. The control method includes the following two steps. A first scanning signal is transmitted to a scanning electrode (Gmxe2x88x921) that is scanned earlier than the target scanning electrode (Gm) in the same video frame. This first scanning signal can turn on the recycling switch to redistribute the charges on the first display electrode and the second display electrode. The voltage level of the first display electrode or the second display electrode is very close to the common electrode voltage VCOM. When the scanning electrode (Gm) is scanned, the data driver can drive these display cells from the common electrode voltage VCOM to the corresponding video signal levels.